The reality is that you need to balance the overall client needs definition with their perception horizon. Agile can be used as a lazy approach to the needs definition because it allows adaption as you go. But if its not applied an controlled correctly it can easily lead to chaos. Waterfall on the other hand fixes the requirement at the beginning on the assumption that these are constants the reality is somewhere in between. The cycle or methodology has to match the client and the project not vice versa. The further out the delivery horizon is the more critical the method becomes as very few variables can remain constant over long periods of time. On the other hand using the customers indecision or requirement for flexibility as a reason can just as easily result in lack of context-poor business case definition-and a broad acceptance of implied results ie you develop what you think the customer wanted/asked for and then fight it out when its wrong( or accept defeat and re do). The method has to start with a plan not the other way round.